This invention relates to a superconducting switching circuit which operates at very low temperatures, more particularly to a counter circuit using a quantum flux parametron (hereinafter "QFP") which is a parametron type switching circuit using Josephson junctions and which is described in Harada, Y. et al., "Basic Operations of the Quantum Flux Parametron", IEEE Trans. on Magnetics, vol. MAG-23, No. 5, September 1987, pp. 3801-3807.
The QFP is a switching circuit based on a new operation concept, using a dc magnetic flux quantum as a signal medium. The QFP is very adequate for a high speed digital circuit element because it operates at a high speed and dissipates little power.
A flip-flop circuit using the QFP has been disclosed in the proceedings for the 1984 Josephson Symposium of the Institute of Physical and Chemical Research, page 99. The flip-flop circuit comprises a combination of an rf-SQUID and the QFP, the rf-SQUID holds data and the QFP reads out the data from the rf-SQUID. Counters play an important role in the logic elements for digital circuits. Especially, a high speed counter capable of counting the number of pulses at a high speed can be used in many applications, that is, not only in computers but also in analog-digital converters. In principle, the counter circuit can be constructed from a flip-flop circuit and a combinational logic circuit. According to this principle, the counter circuit can be constructed by the use of the QFPs, namely, the counter circuit is constructed from the flip-flop circuit and a combinational logic circuit comprising multiple QFPs. However, since the QFP is a circuit excited by a poly-phase ac current, if the counter circuit is constructed using only AC activated QFPs, some problems arise as described below.
1. Since the combinational logic circuit and the flip-flop circuit operate simultaneously with a poly-phase ac current, the ac exciting signal needs to be a high-frequency signal and the phase delay of the exciting signal should be precisely controlled to operate the circuits at a high speed. PA1 2. Since input pulses are counted simultaneously with the high-frequency ac exciting current, randomly incoming pulses can not be counted.